1. Field of the Invention
The present invention relates to inter-cell interference (ICI) in a mobile communication system, and more particularly, to a method for controlling ICI by transmitting uplink control information to a terminal of a neighboring cell based on ICI amount information measured at a base station, and a mobile communication system employing the method.
2. Discussion of the Background
Mobile communication technology includes a communication device useable by a user in a moving object, such as a train, a ship, or an airplane, or while the user is walking.
FIG. 1 illustrates a cell structure of a mobile communication system in which a mobile communication device may be used according to the conventional art. As shown in FIG. 1, a mobile communication system includes base stations 111, 112, and 113, and mobile communication devices, which may be referred to as mobile stations or a mobile communication terminals (hereinafter, referred to individually as “terminal”) 121, 122, and 123. Each base station 111, 112, and 113 provides mobile communication services to a respective wireless communication area, called a “cell”. Each terminal 121, 122, and 123 may be located in a cell corresponding to one of the base stations 111, 112, and 113, and respectively receives mobile communication service via the base stations 111, 112, and 113.
In this instance, a base station 111, 112, or 113 in a cell may be affected by multiple access interference corresponding to signal interference from a terminal located within the cell, and inter-cell interference (ICI) corresponding to signal interference from a terminal located in a neighboring cell.
Orthogonal frequency division multiplexing (OFDM) technology has been developed, and may be able to reduce multiple access interference. However, ICI, and in particular, ICI of an uplink channel has not been solved through the use of OFDM.
In many mobile communication systems, a terminal near a cell boundary may have signal distortion due to the ICI. Accordingly, for secure data transmission, channel coding is performed with an extremely low channel coding rate and then data is transmitted. For example, a portable internet Wireless Broadband (WiBro) standard uses a coding rate of 1/12.
Various solutions have been proposed for reducing ICI problem. FIG. 2 illustrates an example of a frequency band allocation method based on fractional frequency reuse (FFR) according to the conventional art.
Referring to FIG. 2, terminals located in the centers of a cell (1) 221 (cell 221), a cell (2) 222 (cell 222), and a cell (3) 223 (cell 223) use the same frequency band (210). However, a terminal near a cell boundary may not use one predetermined frequency band from among three frequency bands 211, 212, and 213, or may use the predetermined frequency band at a lower power to avoid frequency duplication with a neighboring cell. For example, a terminal near a boundary of cell 221 may not use a first fractional bandwidth 211 or may use the first fractional bandwidth 211 with a lower power. Also, another terminal near a boundary of cell 222 may not use a second fractional bandwidth 212 or may use the second fractional bandwidth 212 with a lower power. Also, still another terminal near a boundary of cell 223 may not use a third fractional bandwidth 213 or may use the third fractional bandwidth 213 with a lower power. As a result, the terminal may reduce ICI, but a frequency reuse factor for a terminal located in an outer boundary of a cell is reduced to ⅔ without regard for the actual use of the three frequency bands prior to avoiding frequency duplication with a neighboring cell.
FIG. 3 illustrates another frequency band allocation method for ICI reduction according to the conventional art. Referring to FIG. 3, a cell is divided into a central area (a white area) and an outer area (a shaded area). In this instance, frequency bands are allocated so that a mobile terminal of the central area may use a common frequency band with neighboring cells, and a terminal of the outer area may use a frequency band that is not used in neighboring cells.
Specifically, a cell (2) 302 (cell 302), a cell (3) 303 (cell 303), a cell (4) 304 (cell 304), a cell (5) 305 (cell 305), a cell (6) 306 (cell 306), and a cell (7) 307 (cell 307) neighbor a cell (1) 301 (cell 301). A first frequency band is allocated to an outer boundary of the cell 301 and is marked in black, but is not duplicated with a second frequency band and a third frequency band allocated to outer boundaries of the cells 302, 303, 304, 305, 306, and 307. Also, the cells 302, 304, and 306, which are allocated with the second frequency band and have outer boundaries marked in dots, are spaced apart from each other. Also, the cells 303, 305, and 307, which are allocated with the third frequency band and have outer boundaries marked in diagonal lines, are spaced apart from each other. Specifically, the ICI reduction scheme shown in FIG. 3 may allocate a frequency band that is not used by neighboring cells in an outer boundary with more severe ICI, thereby reducing the ICI.
In addition to the ICI reduction schemes described with reference to FIG. 2 and FIG. 3, various types of ICI reduction schemes have been proposed. The ICI reduction schemes are commonly based on an idea of ICI coordination and avoidance that limits frequency use time or frequency resources for a terminal located in an outer boundary of a cell.
However, the ICI reduction schemes based on ICI coordination and avoidance, including the FFR scheme, have many problems.
First, in practice, a cell area may have a significantly distorted shape that is different than a theoretical hexagonal cell arrangement. Accordingly, it may be more difficult to define the outer boundary and then separately manage frequency bands for the central area and the outer boundary.
Second, since an available frequency band is reduced, trunking efficiency may be reduced. Specifically, wireless resources may be exhausted when more terminals are located in the outer boundary of the cell.
Third, in comparison to when the same entire frequency band is used in all cells, frequency hopping may be reduced. Accordingly, frequency diversity effect may be reduced, and thus a multi-path signal may not be effectively processed.
Fourth, since a frequency band is allocated to a terminal located in an outer boundary of a cell based on a relation with a neighboring cell, flexible cell planning may be difficult. For example, when adding an additional base station and an additional cell between existing cells, a new frequency band should be allocated to cells adjacent to the additional cell, and this may require the modification of the cell planning.
Fifth, a portion of the frequency band may be unused even if there is no ICI. Accordingly, wireless resources may not be effectively managed.
Finally, in the conventional ICI reduction schemes as described above, an upper layer service control point (SCP) or a mobile switching center (MSC) should be in charge of cell planning and coordination for the base stations. However, this is inconsistent with ALL-Internet Protocol (IP), which is the trend for the next generation communication network.
Accordingly, there is a need for a new technology to control uplink resources of a terminal based on ICI amount information measured at a base station.